[unified_analytics] Add DashToolKind and shared-component DashTool values #2699

Part of the effort to record both tool (logical source of an event) and rootTool (the user's entry point) on every Dash analytics event.

• Add a DashToolKind { tool, sharedComponent } enum, plus a kind field on DashTool (default DashToolKind.tool) and bool get canBeRootTool.
• Add shared-component values: analysisServer ('analysis-server'), dartMcpServer ('dart-mcp-server'), skillsCli ('skills-cli').
• Make topLevelTool() reject shared-component labels. Keep it as is, then either deprecate it or have it delegate to resolveRootTool.

Done when: the new values exist, canBeRootTool returns false for shared components, and tests pass.